1 Answer
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Those are not abbreviations but contractions. If the text is reporting dialogue, then it's usual to use them. Whether a writer uses contractions otherwise depends on the context. They perhaps appear more frequently in informal than formal writing. However, Pam Peters in her entry on contractions in 'The Cambridge Guide to English Usage' writes, with her customary good sense :

The writers of formal documents may feel that they undermine the
authority and dignity of their words. But the interactive quality of
contractions lend to a style is these days often sought, in business
and elsewhere. They facilitate reading by reducing the space taken up
by predictable elements of the verb phrase, and help to establish the
underlying rhythms of prose.

What time frame is "these days", though? I can imagine that at some time in the past, people would rebel against the stricter rules of yesterday, but such trends tend to vary with time.
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TLPOct 18 '11 at 9:57

The Cambridge Guide was published in 2004.
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Barrie EnglandOct 18 '11 at 14:49